1. Field of the Invention
The present invention relates to magnetic disk drive systems. Such systems include one or more magnetic disks, with each disk containing a plurality of concentric data tracks. In order to read information from the disks, a magnetic transducer assembly is radially moved with respect to the disks to a selected data track. One of the critical performance factors of a disk drive system is its access or seek time, i.e., the time it takes to move the transducer assembly from a starting position to a selected destination track. The present invention is directed to a system for reducing the access time in a disk drive assembly.
2. Description of the Prior Art
Most magnetic disk systems currently in use employ a velocity servo system in which for each seek operation, a velocity profile is generated by means of a microprocessor and provides a target velocity as a function of the distance from a designation track. The servo system operates to maintain the velocity of the transducer assembly as close as possible to the velocity profile. Velocity control systems of this type are disclosed in U.S. Pat. Nos. 4,030,132 to Iftikar et al., 4,103,314 to Case, 4,217,612 to Matla et al., 4,237,502 to Erickson, Jr., et al., 4,314,291 to Oda et al., 4,333,117 to Johnson, and 4,396,959 to Harrison et al.
Each of the above patents discloses a system which includes some means of minimizing the access time. In the Iftikar patent, a compensatory signal is provided to the velocity servo control in order to cause the actual motor velocity to follow the program velocity profile more accurately. In Case, position feedback is used in conjunction with velocity control in order to cause the transducer to more accurately follow the predetermined velocity profile. In Oda, a seek operation is performed prior to normal operation in order to determine characteristics of the actuator which is used to move the transducer and the velocity detector which is used in the feedback loop. The determined values are used to automatically adjust the operation of the device in order to compensate for changes in the characteristics of the actuator or velocity detector. The Johnson patent employs a system for digitally determining the head velocity, with the digital determination being compared with a stored velocity profile. The digital determination of velocity is accomplished by measuring the time between successive track crossings.
The Harrison patent employs a first increment of maximum acceleration, a second increment of maximum deceleration and a third increment in which the command amplitude follows a progressive analog staircase which is reset with each detected track crossing. In Erickson, a velocity profile is employed which causes the transducer to creep to the destination track so as to avoid overshoot.